Post-larvae of an intertidal barnacle (Elminius modestus) grew more slowly under ocean acidification conditions, but there were no impacts on its shell calcium content and survival by either ocean acidification or warmer temperature. were observed in high CO2 but there were no impacts on shell calcium content and survival by either elevated temperature or CO2. Post-larvae of another species of intertidal barnacle (Balanus balanoides) exhibited no clear alterations in growth rate but did show a large reduction in shell calcium content and survival under ocean acidification and warmer temperature. The findings suggest that a decrease of 0.4 pH units alone would not affect the survival of young barnacles in the first month after they settle onto rocks. However, if this change in pH were accompanied by a 4–5°C increase in temperature, it could affect the survival and shell structure of the young barnacles. (Laboratory study)